Amplification optical fiber and fiber laser device using the same

An amplification optical fiber comprising:

• a core;
• and a clad surrounding the core, the amplification optical fiber allowing light having a predetermined wavelength to propagate in at least an LP01 mode and an LP11 mode, wherein
• the core includes a first core portion covering a center area of the core and a second core portion surrounding the first core portion,
• a refractive index of the first core portion is higher than that of the clad,
• a refractive index of the second core portion is higher than that of the first core portion,
• when light of the LP01 mode and light of the LP11 mode are standardized by power, in the core, an active element that stimulates to emit light of the predetermined wavelength is doped at a higher concentration in at least a part of an area where power of light of the LP01 mode is larger than that of light of the LP11 mode than at least a part of an area where the power of light of the LP11 mode is larger than that of light of the LP01 mode to satisfy the formula:
• ∫0∞n(r)×{I01(r)−I11(r)}rdr>0
• wherein, r represents a distance from the center in the radial direction of the core, Ioi(r) represents the power of light of the LP01 mode at the distance r from the center in the radial direction of the core, I11(r) represents the power of light of the LP11 mode at the distance r from the center in the radial direction of the core, and n(r) represents an additive concentration of the active element at the distance r from the center in the radial direction of the core.